//https://leetcode.cn/problems/climbing-stairs/?envType=study-plan&id=dong-tai-gui-hua-ru-men&plan=dynamic-programming&plan_progress=y5pmr12
int climbStairs(int n) {
    int a = 1;
    int b = 2;
    int c = a + b;
    if (n == 1 || n == 2)
    {
        return n;
    }
    while (n > 2)
    {
        c = a + b;
        a = b;
        b = c;
        --n;
    }
    return c;
}